1. Field of the Invention
The present invention relates generally to an improved electrochemical detector cell, an electrochemical detection method, and a pyrolysis furnace for use in electrochemical detection. More particularly, the invention relates to such a cell and method which is especially adapted for gravimetric flow. The invention further relates to such a cell and method that is usable for potentiometric, coulometric or conductimetric electrochemical detection.
2. Description of the Prior Art
Before 1950, it was common to determine the elemental composition of organic samples by combusting them and measuring the masses of simple compounds, such as carbon dioxide and water, produced. This method was called microcombustion analysis.
During the 1950s, gas chromatography (GC) was discovered, so there was a lot of research in the late 1950s and early 1960s on GC instrumentation, including sensitive detectors. Thermal conductivity detectors were soon followed by more sensitive and selective detectors, such as hydrogen flame ionization, electron capture, argon ionization, mass spectrometric and several electrochemical detectors. The first electrochemical detectors for use in chromatography were described in U.S. Pat. No. 3,032,493, issued May 1, 1962 to Coulson and Cavanagh. The use of the Coulson and Cavanagh detector with GC is described in Coulson et al., "Microcoulometric Gas Chromatography of Pesticides", Agricultural and Food Chemistry, 8, 399-402 (Sept:Oct. 1960) and Coulson, "Electroanalytical Instrumentation", ISA Fall Instrument-Automation Conference, Preprint No. 181-LA-61 (Sep. 11-15, 1961).
This detection system contained essentially all of the important features of the electrolytic conductivity detectors that were subsequently developed, including:
1) An effluent gas stream from a gas chromatograph. PA1 2) A pyrolizer to convert organic compounds to simple substances, such as carbon dioxide, hydrogen chloride, ammonia and water. PA1 3) A supply of liquid supporting electrolyte. PA1 4) A contact zone to transfer electrolytes from the gas phase to the liquid phase. PA1 5) Electrodes in contact with the liquid phase. PA1 6) Means for measuring the electrolyte concentration in the liquid phase. PA1 7) A gas-liquid separator for an electrolytic conductivity detector.
In microcoulometry, the means for measuring is based on potentiometry and titrant generation. In electrolytic conductivity, this means is simply measurement of the ohmic resistance between two electrodes.
U.S. Pat. No. 3,158,466, issued Nov. 24, 1964 to Sternberg was the first to report on the use of electrical conductivity in gas chromatography of halogenated organic compounds. The Sternberg system has all of the above elements for the microcoulometric detector. Sternberg did not disclose a structure for gas-liquid separation, but did suggest that electrolytes other than water may be used to achieve better sensitivity.
Almost simultaneously, Piringer, et at. reported the development of an electrolytic conductivity detector for GC in "Construction and Operation of the Electrolytic Conductivity Detector," J. Chromatog. 8,410 (1962). They added an additional feature to the six listed above:
U.S. Pat. No. 3,309,845, issued Mar. 21, 1967 to Coulson, discloses an electrolytic conductivity detector utilizing a capillary in the gas-liquid contact zone of 1 to 0.1 mm inside diameter. With the 1 mm capillary, a liquid flow rate of approximately 1 ml. per minute was commonly used, with lower flow rates if the contact zone had a pumped liquid flow. This detector was provided in a commercial product by Tracor. Western Scientific Associates continues to market this detector under license from Tracor, but with the substitution of a modified pump flow controller.
U.S. Pat. No. 3,649,498, issued Mar. 14, 1972 to Pretorious et al. discloses a liquid and gas chromatographic detector cell in which a variety of solvents, including the lower alcohols, in addition to water are used as a carder liquid in the detector cell.
U.S. Pat. Nos. 3,934,193 and 4,032,296, issued Jan. 20, 1976 and Jun. 28, 1977 to Hall, repeat much of the above teaching and disclose unitized detector cells with structure for the physical separation of gas and liquid phases from samples being measured by the cells.
U.S. Pat. No. 4,440,726, issued Apr. 3, 1984 to Coulson discloses a reduced volume, all capillary detection cell suitable for potentiometric and coulometric detection cells. The present invention is a modification of that detection cell.
U.S. Pat. No. 5,019,517, issued May 28, 1991 to Coulson discloses a pyrolysis furnace which incorporates electrodes for detecting ion currents. There is a continuing need for improvement of pyrolysis furnaces, especially for supplying gases incorporating substances to electrochemical detection cells separate from the furnace.